1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising a pulse width modulated demand limit circuit for enhancing power management during spin-down.
2. Description of the Prior Art
When the disk spins down in a disk drive it is important to park the head before the air bearing dissipates to prevent damage to the head and/or the disk. In disk drives wherein the head is parked in a landing zone on the disk, it is also important to brake the spindle motor as quickly as possible to minimize head wear. Conventionally, when power is removed from the disk drive the head is parked using the back EMF (BEMF) voltage present in the spindle motor windings due to the angular momentum of the spindle and disk assembly. The current induced by the BEMF charges a capacitor to thereby generate an internal supply voltage with limited current capability. This internal supply voltage is applied to a voice coil motor (VCM) to park the head. Once the head is parked, a braking torque is applied to the spindle motor to stop it from rotating as quickly as possible in order to minimize head wear. This limited internal supply must provide enough current to allow all shutdown activities to take place, including the sequencer and controls necessary to make the shutdown events happen properly.
If the internal supply voltage drops below a certain threshold for an extended period of time during a power failure mode, the analog and digital circuitry which control the power down sequence may malfunction or shut down. This can typically occur if the VCM driver draws an excessive amount of current from the internal supply voltage. For example, if the head is in the middle of a seek operation when power failure occurs, the VCM driver may draw excessive current in order to decelerate the head. In another example, the VCM driver may draw excessive current in order to prevent the head from bouncing away from the parking latch at the end of the park operation. Either of these events may pull down the internal supply voltage beyond a safe level, thereby rendering the power down operation questionable.
This problem has been addressed in the prior art by designing disk drives with efficient spindle motors capable of providing sufficient BEMF voltage and current during power failure. However, this increases the overall cost of the disk drive since the design restrictions on the spindle motor both raise its costs and put stringent requirements on the design of the rest of the disk drive.
Another known technique which alleviates this problem is to employ a “boost” circuit for “boosting” the internal supply voltage by periodically shorting the spindle motor windings. For example, U.S. Pat. No. 5,504,402 discloses a boost circuit for boosting the internal supply voltage by periodically grounding the spindle motor windings using a grounding switch. When the grounding switch is periodically turned on (grounded), a current builds in the spindle motor windings due to the inductance and the BEMF. During the off or open portion of the periodic short cycle, the current stored in the spindle motor windings charges a capacitor which then provides a crudely filtered internal supply voltage. When the internal supply voltage reaches a predetermined level, the grounding switch is left off in order to prevent overcharging the capacitor to a voltage that is higher than needed. Although the boost circuit increases the available power so that less efficient spindle and VCM motors may be employed, further improvements are attainable.
Head wear is also reduced by unlatching the head early in the spin-up process after the spindle motor reaches a sufficient RPM to enable the head to fly over the disk. This means the heads are unlatched at about 60% of the operating RPM. Since power can be lost at any time, any RPM at which the heads are unlatched must also generate enough voltage and power to allow the heads to park.
Yet another problem identified with prior art disk drives during a normal spin-down operation is that the head is typically positioned over the landing zone on the disk while the disk is still rotating at a high RPM resulting in undesirable head wear. The landing zone is typically textured to reduce the stiction force during spin-up; however, this extremely hard textured surface also wears on the head during spin-down. Because prior art techniques position the head over the landing zone while the disk is still spinning at a high RPM, head wear increases due to the increased time to brake the spindle motor. The prior art boost circuit alleviates this problem somewhat by increasing the internal supply voltage, thereby enabling head parking at a lower RPM which reduces the braking time while the head is over the landing zone. However, further improvements are attainable.
Spindle motors designed to operate at a higher RPM will often be used in disk drives having a lower operating RPM. This creates a low voltage tolerance problem since the back EMF from the spindle motor is proportional to design RPM, not to actual operating RPM. Delaying head parking during spin-down using these reduced voltage spindle motors to minimize head wear causes additional sag in the internal supply voltage due to even lower RPM.
There is, therefore, a need to improve upon prior art techniques for generating an internal supply voltage used to park the head in a disk drive during spin-down. In particular, there is a need to protect against the internal supply voltage from dropping below a safe level in order to maintain proper operation of the digital and analog circuitry responsible for the power down sequence. Further, there is a need to improve power management so that head parking is viable at a lower RPM to reduce head wear.